1. Field of the Invention
The present invention relates to printed wiring boards and, in particular, to layers therefor having increased, high density conductors and feed-through vias.
2. Description of the Prior Art
The number of conductors, which can be successfully positioned on a conventional layer of a multilayer printed wiring board, is limited primarily by the large pads of copper which provide terminations for the conductors. Each pad, and even more so clusters of pads, effectively block channels or locations for the conductors, making it necessary to move the conductors to other less dense layers in the printed wiring board in order to make a connection. The functions of such pads conventionally are electrical and mechanical. The electrical function is to make an electrical connection at the pad, such as to a feed-through or via in the layer, and for this function, it could be much smaller. The purpose of the mechanical function enables the feed-through hole to be completely surrounded in a worst tolerance condition and to prevent etchants from attacking the metal in and adjacent to the hole during board fabrication. Thus, the pad must be made very large, typically 20 mils or more larger than that needed to satisfy its electrical function.
Specifically, the conventional process of forming an etched feedthrough hole or via begins with a blank comprising a core of dielectric material which is doubly cladded with copper. To form the feed-through, holes are drilled through the blank and made conductive with sequential deposits of electroless and electroplated copper. Photoresist material is then formed in a pattern over all exposed metal on the blank except for the surface conductors, lands and holes. Finally, additional copper, followed by tin-lead solder, is electroplated about and into the plated hole and on the conductive material about the plated hole to form a land area. The photoresist is removed and, using the solder as an etch resistant material, the exposed cladding is etched to form conductors on the surfaces of the blank and to form it into a printed wiring board layer. It is obvious, therefore, that the copper and solder plated pad functions both as a means for electrical connection as well as for protection against etching of the copper in the hole. If these plated through holes were vias in an internal layer of a multilayer printed wiring board, the solder plate would be stripped prior to lamination in order to enhance bonding. As an alternate to plating solder around and into the holes, the holes may be tented with photoresist film before the etching operation.
The size of the land area, which is used to protect the copper deposited in and about the feed-through hole during etching, is determined by three major factors. First, there must be sufficient tolerance, typically of 10 mils, to afford a proper registration from one photoresist pattern to another. For example, the holes must be surrounded by exposed copper, even though the resist image will be shifted one way or the other due to fabrication tolerances. Second, there must be a tolerance in the location in the various holes to accomodate any off-center hole and, for this purpose, the holes are made approximately 6-10 mils larger than the hole diameter. Third, the land must be further enlarged to protect the copper in the hole from being etched; the predetermined tolerance therefor being about 4-10 mils beyond the hole diameter. The sum of these tolerances and requirements provide a land area which is 20-30 mils larger than the hole. Since the smallest hole diameter typically is 13.5 mils in diameter, the smallest sized land area is 33.5-43.5 mils, or two and one-half to three times the size of the hole.
As a result of such large land areas, the conductor density is limited, and the spacing between plated through holes is limited to approximately 50 mil centers, a standard in the industry. If it were desired to increase the number of conductors, then more printed wiring board layers would be required to interconnect any given set of component terminations. It is preferable to reduce the number of layers and to increase the density within any given layer.
More recently, hermetic leadless carriers are increasingly being used, with the aim of terminations on 20 to 25 mil centers. Such decreased spacing creates a requirement for even higher density of printed wiring board layers than is achievable using conventional techniques.